Kees Cook
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to Jann Horn, Kees Cook, Greg KH, Linus Torvalds, Seth Jenkins, Andy Lutomirski, Petr Mladek, Andrew Morton, tangmeng, Guilherme G. Piccoli, Tiezhu Yang, Sebastian Andrzej Siewior, Eric W. Biederman, Arnd Bergmann, Marco Elver, Dmitry Vyukov, Peter Zijlstra, Juri Lelli, Vincent Guittot, Dietmar Eggemann, Steven Rostedt, Ben Segall, Mel Gorman, Daniel Bristot de Oliveira, Valentin Schneider, Andrey Ryabinin, Alexander Potapenko, Andrey Konovalov, Vincenzo Frascino, Luis Chamberlain, David Gow, Paul E. McKenney, Jonathan Corbet, Baolin Wang, Jason A. Donenfeld, Eric Biggers, Huang Ying, Anton Vorontsov, Mauro Carvalho Chehab, Laurent Dufour, Rob Herring, linux-...@vger.kernel.org, kasa...@googlegroups.com, linu...@kvack.org, linu...@vger.kernel.org, linux-h...@vger.kernel.org
From: Jann Horn <
ja...@google.com>
Many Linux systems are configured to not panic on oops; but allowing an
attacker to oops the system **really** often can make even bugs that look
completely unexploitable exploitable (like NULL dereferences and such) if
each crash elevates a refcount by one or a lock is taken in read mode, and
this causes a counter to eventually overflow.
The most interesting counters for this are 32 bits wide (like open-coded
refcounts that don't use refcount_t). (The ldsem reader count on 32-bit
platforms is just 16 bits, but probably nobody cares about 32-bit platforms
that much nowadays.)
So let's panic the system if the kernel is constantly oopsing.
The speed of oopsing 2^32 times probably depends on several factors, like
how long the stack trace is and which unwinder you're using; an empirically
important one is whether your console is showing a graphical environment or
a text console that oopses will be printed to.
In a quick single-threaded benchmark, it looks like oopsing in a vfork()
child with a very short stack trace only takes ~510 microseconds per run
when a graphical console is active; but switching to a text console that
oopses are printed to slows it down around 87x, to ~45 milliseconds per
run.
(Adding more threads makes this faster, but the actual oops printing
happens under &die_lock on x86, so you can maybe speed this up by a factor
of around 2 and then any further improvement gets eaten up by lock
contention.)
It looks like it would take around 8-12 days to overflow a 32-bit counter
with repeated oopsing on a multi-core X86 system running a graphical
environment; both me (in an X86 VM) and Seth (with a distro kernel on
normal hardware in a standard configuration) got numbers in that ballpark.
12 days aren't *that* short on a desktop system, and you'd likely need much
longer on a typical server system (assuming that people don't run graphical
desktop environments on their servers), and this is a *very* noisy and
violent approach to exploiting the kernel; and it also seems to take orders
of magnitude longer on some machines, probably because stuff like EFI
pstore will slow it down a ton if that's active.
[Moved sysctl into kernel/exit.c -kees]
Signed-off-by: Jann Horn <
ja...@google.com>
Signed-off-by: Kees Cook <
kees...@chromium.org>
Link:
https://lore.kernel.org/r/20221107201317...@google.com
---
Documentation/admin-guide/sysctl/kernel.rst | 8 ++++
kernel/exit.c | 42 +++++++++++++++++++++
2 files changed, 50 insertions(+)
diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index 98d1b198b2b4..09f3fb2f8585 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -667,6 +667,14 @@ This is the default behavior.
an oops event is detected.
+oops_limit
+==========
+
+Number of kernel oopses after which the kernel should panic when
+``panic_on_oops`` is not set. Setting this to 0 or 1 has the same effect
+as setting ``panic_on_oops=1``.
+
+
osrelease, ostype & version
===========================
diff --git a/kernel/exit.c b/kernel/exit.c
index 35e0a31a0315..892f38aeb0a4 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -72,6 +72,33 @@
#include <asm/unistd.h>
#include <asm/mmu_context.h>
+/*
+ * The default value should be high enough to not crash a system that randomly
+ * crashes its kernel from time to time, but low enough to at least not permit
+ * overflowing 32-bit refcounts or the ldsem writer count.
+ */
+static unsigned int oops_limit = 10000;
+
+#if CONFIG_SYSCTL
+static struct ctl_table kern_exit_table[] = {
+ {
+ .procname = "oops_limit",
+ .data = &oops_limit,
+ .maxlen = sizeof(oops_limit),
+ .mode = 0644,
+ .proc_handler = proc_douintvec,
+ },
+ { }
+};
+
+static __init int kernel_exit_sysctls_init(void)
+{
+ register_sysctl_init("kernel", kern_exit_table);
+ return 0;
+}
+late_initcall(kernel_exit_sysctls_init);
+#endif
+
static void __unhash_process(struct task_struct *p, bool group_dead)
{
nr_threads--;
@@ -874,6 +901,8 @@ void __noreturn do_exit(long code)
void __noreturn make_task_dead(int signr)
{
+ static atomic_t oops_count = ATOMIC_INIT(0);
+
/*
* Take the task off the cpu after something catastrophic has
* happened.
@@ -897,6 +926,19 @@ void __noreturn make_task_dead(int signr)
preempt_count_set(PREEMPT_ENABLED);
}
+ /*
+ * Every time the system oopses, if the oops happens while a reference
+ * to an object was held, the reference leaks.
+ * If the oops doesn't also leak memory, repeated oopsing can cause
+ * reference counters to wrap around (if they're not using refcount_t).
+ * This means that repeated oopsing can make unexploitable-looking bugs
+ * exploitable through repeated oopsing.
+ * To make sure this can't happen, place an upper bound on how often the
+ * kernel may oops without panic().
+ */
+ if (atomic_inc_return(&oops_count) >= READ_ONCE(oops_limit))
+ panic("Oopsed too often (oops_limit is %d)", oops_limit);
+
/*
* We're taking recursive faults here in make_task_dead. Safest is to just
* leave this task alone and wait for reboot.
--
2.34.1